Typical image forming processes involving electrophotography and electrostatic printing include a developing step in which a photo conductive insulating layer is uniformly electrified, an electrical latent image is formed by dissipating the charge on the exposed sections once the dielectric layer has been exposed to light and a fine powder toner carrying a charge is adhered to the latent image to render it visible, a transfer step in which the resulting visible image is transferred to a transfer material such as transfer paper, and a fixing step for permanently fixing it by heat or pressure.
The toners and toner binder resins used in electrophotography and electrostatic printing must have a number of different properties for each of these steps. For example, in order to adhere the toner to the electrical latent image in the developing step, the toner and toner binder resin must maintain an amount of charge suitable for copying machines without being affected by the temperature or humidity of the surrounding environment. Also, in the fixation step using a heated roller fixing system, they must have an anti-offset property so as not to stick to heated rollers, while having satisfactory fixability onto the paper. Blocking resistance is also required so that the toner does not undergo blocking during storage in the copying machine.
Styrene-acrylic resins have been widely used as toner resins in the past, and especially linear resins and crosslinked resins. In the case of linear resins, a high molecular weight polymer is blended with a low molecular weight polymer to improve the toner fixing property and anti-offset property. However, with toners which employ such resins, for instance when the fixing property is improved, the melt viscosity of the resin decreases and the anti-offset property of the toner is lowered, and it is therefore difficult to obtain toner with an adequate balance between the two. Attempts have therefore been made to improve the balance between fixing property and anti-offset property by widening the molecular weight distribution of the resin.
For example, in Japanese Examined Patent Publication No. 63-32182, No.63-32183, No.63-32382 and 3-48506 there are proposed methods for improving the balance between fixing property and anti-offset property by widening the molecular weight distribution of the resin, by means of a blend of high molecular weight polymers and low molecular weight polymers with specific molecular weight distributions. However, it has still not been possible to fully satisfy demands for both the fixing property and anti-offset property by simple blending of high molecular weight polymers and low molecular weight polymers.